This invention relates to the delivery of therapeutic agents to target sites, with provision for binding to the target sites.
The use of an antibody/antigen binding as a means to attach a therapeutic agent to a target site has already been proposed, for example with glucose oxidase as the therapeutic agent. This enzyme catalyses the oxidation of glucose to gluconic acid by molecular oxygen, producing hydrogen peroxide in the process. Hydrogen peroxide is rapidly decomposed in vivo, but if it can be brought close to target cells it does exhibit toxicity to those cells.
As an example of such a proposal, using whole antibodies, Knowles et al J. Clinical Investigation 52 1443 (1973) have described the use of glucose oxidase chemically conjugated to antibodies capable of binding to target cells, thereby targeting the cell killing activity against those cells which it is desired to eliminate selectively. Cell killing, however, was only achieved if other (non-targeted) enzymes were present in the surrounding medium to generate even more toxic species.
WO 89/11866 (Rama Biolink) discloses antibiotic conjugated to anti S. mutans antibodies obtained by immunisation of hens.
The use of fragments of antibodies, rather than whole antibodies, to effect immunological binding, has been proposed in various documents including EP-A-175560 and EP-A-315364 which mention chemical conjugation to antibody fragments.
Techniques for expressing variable domains in bacteria, and hence producing Fv fragments of antibodies, are described by Saikl et al Science, 230 1350 (1985) and by Oranei et al Procnatl Acad Sci U.S.A. 88 3833 (1989). Production of antibody fragments is also discussed in EP-A-368684 (M.R.C.). This mentions the possibility of producing antibody fragment joined through a linking peptide to a protein having a required function, so that the peptide chain of the antibody fragment, the linking peptide and the functional protein are all expressed as a single fusion protein. Producing such a single protein has the difficulty that the yield of expressed protein may be low.
Expression of antibody fragments with the peptide chain prolonged and terminated by an additional peptide, to act as a linker, is described in our WO 91/08492 published 13 Jun. 1991.